Well in fact AHTS, PSV and R.O.V. support are different types of supply vessels.
Some parts of the job may change, but these different types are quite close each from another.
And you what do you do for now, do you work on ships or studying ?
By the way, there a topic called "master's licence", I didn't read so I don't what they are talking about on it but did you have a look?
http://www.shipsim.com/ShipSimForum/index.php/topic,6058.0/topicseen.html (http://www.shipsim.com/ShipSimForum/index.php/topic,6058.0/topicseen.html)